1. Field of the Invention
The invention relates to a device for machining the surface of a thin-walled hollow cylinder with the aid of a laser beam, particularly for producing a rotary screen which can be used, for example, for printing materials.
2. The Prior State of the Art
The production of a rotary screen is usually based on a metallic hollow cylinder which is thinly coated on its external surface with an organic lacquer and has a very thin wall thickness. The metallic hollow cylinder has a multiplicity of small through-openings which are located very closely adjacent to one another and which are tightly closed by the organic lacquer. To generate a master pattern, the metallic hollow cylinder is clamped with both ends in a device which is similar to a lathe and which has accurate concentricity. A laser beam moved closely parallel to the longitudinal axis of the hollow cylinder is deflected by a deflection mirror, which can be moved axially parallel, in such a manner that it then extends perpendicularly to the surface of the hollow cylinder. This laser beam is focused with the aid of a lens arrangement in such a manner that a focal point of a very small diameter comes to lie accurately on the cylinder surface. If the laser beam is pulsed, the organic lacquer can be removed by vaporization from the surface of the hollow cylinder in accordance with a predetermined master pattern, the laser beam being guided in the axial direction of the hollow cylinder, on the one hand, and the hollow cylinder itself being rotated. In the areas where the lacquer has been removed, the through-openings in the hollow cylinder are then exposed so that viscous means, for example a printing paste, can be pressed through them at a later stage.
When the electronic master pattern is transferred to the surface of the hollow cylinder, its thin cylinder wall must run with sufficiently accurate concentricity because otherwise each running error also causes a faulty position of the engraving, that is to say of the master pattern to be generated.
To achieve accurate concentricity, the hollow cylinder has in the past been pushed onto an accurately concentric mandrel. However, this has the disadvantage that a handling space of the order of magnitude of the hollow cylinder or of the order of magnitude of the screen length must remain free in the axial direction of the device. In addition, it is required to exchange the concentric mandrel for a mandrel with another diameter when the diameter of the rotary screens is changed, which represents a considerable problem in the refitting work because of the size and the weight of these parts. On the other hand, it is already known to push a ring onto the hollow cylinder to be machined or to be engraved, the inside diameter of which ring is larger by only a few tenths of a millimeter than the outside diameter of the hollow cylinder. Friction causes the ring to be entrained by the hollow cylinder and to revolve with it. The ring is carried along with the lens system which focuses the laser beam so that it is initially located at the left-hand end of the hollow cylinder and then slowly moves together with the lens system to the other or right-hand end of the hollow cylinder. The disadvantage here is that the laser beam moves laterally past the ring. Due to the very thin wall of the hollow cylinder, it is therefore no longer sufficiently damped in the area of the laser beam so that vibrations in this area, particularly diaphragm oscillations, can still lead to considerable errors during the engraving of the hollow cylinder with the aid of the laser beam.